1. Field of the Invention
The present invention relates to the placement of large multi-ton prefabricated deck packages or "rigs" in an offshore environment upon a usually partially submerged jacket that extends between the seabed and the water surface. Even more particularly, the present invention relates to the use of a moving lifting assembly which is preferably barge supported that can place a very large deck package upon an offshore jacket foundation without the use of enormous lifting booms such as form a part of derrick barges, offshore cranes, and the like.
2. General Background
In the offshore oil and gas industry, the search for oil and gas often involves a marine environment. Sometimes the search goes many miles offshore, and drilling takes place into the seabed in many hundreds of feet of water depth. The problem of drilling oil wells offshore and then producing these wells has been solved in part by the use of enormous underwater fixed platform structures with foundations which are mostly submerged, but usually extending a number of feet above the water surface. Upon this foundation (or "jacket" as it is called in the art) there is usually placed a very large prefabricated rig or deck platform. The term "deck platform" as used herein should be understood to include any of a large variety of prefabricated structures which are placed on an offshore jacket foundation to form a fixed offshore platform. Thus, a "deck platform" can include, e.g. a drilling rig, a production platform, a crew quarters, living quarters, or the like. The supporting jacket is usually a very large multi-chord base formed of multiple sections of structural tubing or pipe which are welded together. Such jackets have been used for a number of years for the purpose of supporting large deck platforms in an offshore environment.
The jacket or foundation is usually prefabricated by welding on land in a fabrication yard, preferably beside a navigable waterway. Thus the completed jacket can be placed upon a large transport barge so that it can be moved to the drill site where it will be placed on the ocean floor but extending to the water surface. As an example, an offshore jacket can be several hundred feet in length. The size of the jacket is of course a function of the depth of water in which the rig will be placed. Thus, a five hundred (500) foot water depth at the drill site (or production site) will require a jacket which is approximately 500-550 feet tall. The jacket is usually partially submerged, with a small upper portion of the jacket extending slightly above the water surface. An offshore jacket as described and in its position on the seabed can be seen, for example, in the Blight, et al Pat. No. 4,252,469 entitled "Method and Apparatus for Installing Integrated Deck Structure and Rapidly Separating Same from Supporting Barge Means." Specifically, FIGS. 1, 2, and 3 of the Blight, et al patent show an offshore jacket on the seabed.
A small upper portion of the jacket extends above the water surface. This exposed portion of the jacket is the portion upon which the "deck platform" is placed and supported by. This upper portion of the jacket is usually equipped with a number of alignment devices which enhance the proper placement of the deck package on the jacket. Such alignment devices are referred to variously as stabbing eyes, sockets, or the like. The use of such alignment devices, sockets, or stabbing eyes can be seen in the Blight, et al Pat. Nos. 4,252,468 and 4,252,469 as well as in the the Karsan U.S. Pat. No. 4,242,011. For purposes of background and reference, the Karsan patent 4,242,011 is incorporated herein by reference. The Blight, et al U.S. Pat. Nos. 4,252,469 and 4,252,468 are likewise each incorporated herein by reference.
Deck platforms can be extremely large and have correspondingly heavy weights. For example, it is not uncommon for a deck platform such as a drilling rig, crew quarters, production platform or the like to be between 500 and 5,000 tons gross weight. Such enormous load values present significant problems in the placement of deck platforms on offshore jacket structures. Firstly, the placement is done entirely in a marine environment. While the jacket can be laid on its side and/or floated into position, the quarters itself is not a submersible structure and must be generally supported in an upright condition above the water surface to prevent water damage to the many components which form a part of the drilling or production platform such as electrical systems, wall constructions, and all of the other portions of a typical structure which will be inhabited by individuals and used as oil and gas well drilling or production equipment. The art has typically used enormous derrick barges for the purpose of setting or placing deck packages on jackets in an offshore environment. These derrick barges are large, rectangular barge structures having a high capacity lifting boom mounted at one end portion of the deck of the barge. The barge, for example might be 300-400 feet in length, 50-75 feet in width, and 25-30 feet deep. These figures are exemplary. A derrick barge might have a lifting capacity of for example, 800 tons. For very large structures such as for example, a 1500 ton deck package, two derrick barges can be used, each supporting one side portion of the deck platform with a multi-line lift system supported by an enormous structural boom which extends high into the air above the package during the lift. The boom simply works in the same way as in onshore lifting boom, namely the loadline raises and/or lowers the package into its proper position upon the jacket. While the use of such derrick barges has been very successful in the placing of offshore deck packages on jackets through the years, such derrick barges are generally limited in their capacity to packages of 2,000 tons or less. Further, derrick barges of such an enormous capacity are extremely expensive to manufacture and operate. Many thousands of dollars per hour as a cost of using such a device is not uncommon.
However, when very large loads of, for example 3,000-4,000 tons are involved, the limitation of the derrick barge usually prohibits such a placement on an offshore jacket.
The present invention provides a method and apparatus for the placement of a multi-ton deck package on an offshore jacket. The apparatus includes one or more barges defining a base that supports the large multi-ton load of the deck package. In the preferred embodiment, a horizontally extending truss is mounted on each side of the deck package to be lifted during operation. In the preferred embodiment, two barges are used respectively, each having a preferably variable dimension lift truss on its upper deck surface. The truss preferably includes a member of variable length so that the cross-sectional dimensions of the truss can be varied. The truss forms thus a load transfer between each barge and the deck package to be lifted and placed. Upper and lower connections are formed between the lifting truss and the deck package at respective upper and lower elevational positions. Power is provided, preferably in the form of a winch mounted on the barge for changing the length of the variable length member of the truss so that elevational position of the deck package with respect to the barge can be varied such as during a lowering of the deck package to the jacket foundation. In the method of the present invention, the multi-ton deck package is first transported on a transport barge to the site where it will eventually assist in the drilling oil and/or production of a well. In the preferred embodiment, a lifting assembly is attached to the deck package on generally opposite sides of the deck package and at upper and lower positions. However, it should be understood that the lifting assembly could be attached to the prefabricated deck package prior to transportation to the site. In the latter embodiment, the transport barge and one or more lifting barges could be lashed together and transported to the site by tugs or like tow vessels. One element of the truss-like lifting assembly preferably includes a movable portion which has a variable length. In the preferred embodiment, the movable portion is a winch powered cable which can be extended or retracted between the lift barge and the deck package being lifted. In the preferred embodiment, two lift barges support respectively a pair of truss-like lifting assemblies which in combination with the deck package form an overall truss arrangement. That is, the deck package itself forms a portion of the truss during the lift, and may carry both compression and tension loads. The truss-like lifting assemblies thus support the deck package and elevate it above the surface of the transport barge so that the transport barge can be removed at a support for the deck package. This allows the deck package to be placed vertically above the jacket and aligned with the jacket so that the deck package can be placed upon the jacket by lowering.
The above referenced Blight patents '468 and '469 and the Karsan patent 4,242,011 do relate generally to methods of installing deck structures upon offshore jackets. In Karsan, however, a single transport barge is placed under the deck platform. The deck platform extends laterally and downwardly toward the water line and the upper surface of the jacket, defining a central vessel passageway that is occupied by the transport barge. Unlike the present invention, Karsan does not use truss-like lifting assemblies that are positioned on each side of the deck package being lifted. Further, Karsan doesn't use a truss-like lifting assembly having at least one member of the truss which is of a variable length. With the present invention, this allows a dimensional change in the cross-sectional configuration of the truss with respect to a vertical cross section of the truss thus providing a means of raising and lowering the deck package. These same comments are applicable to the above discussed Blight, et al. patents which also relate to the use of a single transport barge which is placed centrally and under the deck platform to be lifted. Further, Blight, et al. leaves the transport barge in position until the deck platform and jacket have been assembled. This requires in many cases a specially designed jacket and deck platform having a central passageway which will accommodate the vessel under the deck platform and between the peripheral elements of the jacket and the deck platform. This system would not be able to lift the large number of packages which are generally cube shaped and have no central passageway which would accommodate a vessel as described in Blight et al. and Karsan.
An earlier patent, U.S. Pat. No. 2,598,088 issued to H. A. Wilson entitled "Offshore Platform Structure and Method of Erecting Same" discusses the placement of a drilling structure with a barge wherein the legs of the drilling structure are placed while the drilling structure is supported by two barges. The Wilson device does not use truss-like lifting assemblies having variable length portions which are placed generally on opposite sides of the deck package. Rather, Wilson relates to a platform which is floated in place and the support legs are then placed under the floating platform. Thus, in the Wilson reference, an in-place underlying supporting jacket is not contemplated.
The Natvig, et al U.S. Pat. No. 3,977,346 discusses a method of placing a deck structure upon a building site such as a pier. The method includes the pre-assembly of a deck structure upon a base structure on land so that the deck structure extends outwardly over a body of water. Floating barges are provided for supporting the deck structure outwardly of the building site. The deck structure is then transferred to the supportive base structure by means of barges. The Natvig reference uses two barges which are placed on opposite sides of a platform with pedestal type fixed supports forming a load transfer member between the barges and the platform. However, the fixed pedestals of Natvig are unlike the truss-like lifting arrangement of applicant which include movable portions at least one of which can be of a variable length.